1. Field of the Invention
The present invention relates to a developing roller and an image formation apparatus. More particularly, the present invention is concerned with a developing roller which is used for the purpose of visualizing with a developer, an electrostatic latent image preserved on the surface of a latent image preserving body such as a photosensitive drum, in an image formation apparatus such as an electrophotographic apparatus and an electrostatic recording apparatus, including copying machinery, printers, facsimile apparatuses and the like, and which is capable of affording favorable images for a long period of time for the reason that the temperature rise of the roller surface is suppessed, thereby preventing a developer from being adhesively fixed to the roller surface; and also is concerned with an image formation apparatus equipped with the foregoing developing roller.
2. Description of the Related Arts
With regard to an electrophotographic image formation apparatus such as copying machinery, printers, and the like, there is previously known a pressurized developing method as an image formation method which comprises supplying a unary toner (developer) to a latent image preserving body such as a photosensitive body that preserves an electrostatic latent image, and visualizing the latent image by allowing the toner to adhere to the latent image (refer to U.S. Pat. No. 3,152,012 and U.S. Pat. No. 3,731,146).
The pressurized developing method carries out the image formation by bringing a developing roller that supports a toner into contact with a latent image preserving body (photosensitive body) which preserves an electrostatic latent image, and allowing the toner to adhere to the latent image on the surface of the aforesaid latent image preserving body, whereby the developing roller is required to be constituted of an electroconductive elastic body having both electroconductivity and elasticity.
Specifically in the foregoing pressurized developing method the constitution is such that, for instance, as illustrated in FIG. 2, a developing roller 1 is placed between a toner application roller 5 for toner supplying and a latent image preserving body 6 (photosensitive body) preserving an electrostatic latent image; the developing roller 1, the latent image preserving body 6 ( photosensitive body ) and the toner application roller 5 rotate each in the direction of the arrow in FIG. 2, thereby a toner 7 is supplied onto the surface of the developing roller 1 with the toner application roller 5, and is arranged into a uniform thin film by a layer regulation member 8 (layer forming blade); the developing roller 1 rotates in the state that the toner 7 is so arranged, while being in contact with the latent image preserving body 6; and the toner thus formed into a thin film is allowed to adhere to an latent image on the latent image preserving body 6 from the developing roller 1, whereby the aforesaid latent image is visualized. Symbol 9 in FIG. 2 indicates a transfer portion, where a toner image is transferred to a recording medium such as paper. Symbol 10 in FIG. 2 indicates a cleaning portion, where the cleaning blade 11 removes the toner which remains after the transfer on the surface of the latent image preserving body 6.
In such image formation apparatus by using the pressurized developing method as mentioned above, the developing roller 1 is obliged to rotate, while maintaining the state of close contact with the latent image preserving body 6. For this reason, the constitution of the developing roller 1 is such that as illustrated on the schematic cross-section of FIG. 1, a shaft 2 which consists of an electroconductive material such as a metal is equipped on its outside periphery with an electroconductive elastic layer 3 composed of an electroconductive elastic body which is imparted with electroconductivity by blending an electroconductivity imparting agent in elastic rubber such as silicone rubber, acrylonitrile butadiene rubber, ethylene propylene rubber and polyurethane rubber or foam thereof. In addition, a coating layer 4 which is composed of a resin or the like is installed on the surface of the electroconductive elastic layer 3 in order to control electrostatic property and adhesivity for the toner 7, control the force of friction between the latent image preserving body 6 and the layer regulating member 8 (layer forming blade), or prevent fouling of the latent image preserving body 6 due to the elastic body. With regard to the developing roller as mentioned herein-before, in order to assure a bias voltage which comes to be the driving force for transferring the developer preserved thereon to the latent image preserving body, the electric resistivity of the overall developing roller is made to be 104 to 1011 xcexa9, approximately. Further in many cases, for the purpose of facilitating the regulation of the electric resistivity thereof, the specific volume resistance of the electroconductive elastic layer is made low, whereas the specific volume resistance of the coating layer which is composed of a resin is made high. In this case, the specific volume resistance of the coating layer is regulated by incorporating electroconductive powders such as carbon black and a matal oxide in a resin which constitutes the coating layer.
In the case of performing the development of electrostatic latent images by the use of the developing roller such as the above through the pressurized developing method, the surface temperature of the developing roller end portion in particular, is raised by the friction with the toner, toner sealing material or the like, and as a result, there is sometimes caused such phenomenon that the toner is adhesively fixed onto the roller, whereby the roller surface is scraped off. When the hardness of the developing roller surface is increased to enhance the wear resistance as a countermeasure thereaginst, in spite of the enhanced wear resistance of the developing roller itself, in the case of the pressurized developing method, the area of contact between the roller and the latent image preserving body such as a photosensitive body is decreased, thereby making it impossible to carry out favorable development as the case may be. In addition, an excessively high hardness of the developing roller surface often gives rise to a damage to the latent image preserving body. What is more, the excessively high hardness thereof causes a fear of damage to a developer as the case may be because of an overload applied to the developer between the roller and a layer regulating member which is in butt contact with the roller.
Under such circumstances, a general object of the present invention is to provide a developing roller which is used for the purpose of visualizing with a developer, an electrostatic latent image preserved on the surface of a latent image preserving body such as a photosensitive drum, in an image formation apparatus such as an electrophotographic apparatus and an electrostatic recording apparatus, and which suppesses the temperature rise of the roller surface, so that a developer is prevented from being adhesively fixed to the roller surface, wear on the roller surface is suppressed, and cracking, crazing and the like on the roller end hardly occurs, thus enabling to afford favorable images for a long period of time; and an image formation apparatus equipped with the developing roller. Further objects of the present invention will be made obvious from the content of the specification hereinafter disclosed.
In such circumstances, intensive research and development were accumulated by the present inventors in order to solve the problems and thus achieve the above-mentioned objects. As a result, it has been found that the objects are achievable by a roller which is equipped on the outside periphery of a shaft having good electroconductivity with an electroconductive elastic layer having a thermal conductivity of at least a prescribed value. The present invention has been accomplished by the above-mentioned findings and information.
That is to say, the present invention provides a developing roller which comprises a shaft having good electroconductivity and an electroconductive elastic layer formed on the outside periphery of said shaft, supports a developer on its surface to form thin films thereof and in this state, rotates in contact with or in close vicinity to the surface of a latent image preserving body that preserves an electrostatic image on its surface, and thus supplies the developer to the surface of the latent image preserving body so as to visualize an electrostatic image on the surface of the latent image preserving body, said electroconductive elastic layer having a thermal conductivity of at least 0.15 W/mxc2x7K.
Furthermore, the present invention provides an image formation apparatus comprising a latent image preserving body capable of preserving an electrostatic image on the surface thereof and a developing roller which is placed so as to rotate in contact with or in close vicinity to the surface of the latent image preserving body along the surface of the aforesaid latent image preserving body, which supports a developer on its surface to form thin films thereof and which supplies the developer to the surface of the latent image preserving body so as to visualize an electrostatic image on the surface of the latent image preserving body, wherein the above-mentioned developing roller according to the present invention is employed as the developing roller.